Ultracold Atoms Realize Long-Predicted Bethe Strings
Physicists have created and directly observed Bethe strings, unusual quantum states predicted by Hans Bethe in 1931, in an ultracold gas. Cesium atoms cooled to near absolute zero and confined in thousands of one-dimensional tubes formed bound clusters without chemical bonds, including groups of six or more particles. The University of Innsbruck-led team published the results in Nature Communications.
Hans Bethe proposed in 1931 that particles confined to one dimension could form collective bound states. For decades these Bethe strings remained theoretical. The Innsbruck-led team used cesium cooled to billionths of a degree above absolute zero, split into thousands of narrow tubes, and tuned interactions from repulsive to attractive.
The resulting clusters varied in size, some with at least six atoms. When allowed to expand within the tubes, strings collided and stayed intact. Released into three dimensions, they broke apart, converting binding energy into faster motion. This contrast helped confirm their existence. Published in Nature Communications.
This result may mainly affect quantum physicists and theorists by offering a controllable platform to study one-dimensional many-body physics. In the longer term, improved understanding of exotic bound states could inform quantum simulation and future quantum technologies, though practical applications remain speculative. General audiences may see it as a reminder that long-standing theoretical predictions can be tested with ultracold atoms.